Both deletions and insertions occur during the evolution of microbial genomes. Insertions bring in new genes that may be useful for the cell. Deletions

A) may increase fitness of a microorganism by eliminating unneeded genes.
B) always result in a severe loss of fitness for the microorganism.
C) are uncommon because they are usually lethal.
D) always result in a severe loss of fitness but keep microbial genomes compact.